Traditionally, in order to carry out drilling holes smoothly, a combination of a cylindrical core body provided with drilling cutting blades at a peripheral edge portion of an opening and a center drill provided at a center of the core body has been used as a core drill for use in drilling holes with a large diameter.
In such a core drill, first, the center drill drills a center hole, and thereafter, the core body rotates around the center drill positioned in the center hole to drill a large diameter hole which is a target hole around the center hole.
An exemplary construction for the cutting portion structure of the core drill of this type which is for use in drilling holes in concrete, stone, etc, is such that cutting blade chips which are made of cemented carbide or bound with diamond grains are embedded in concave portions formed in an opening end of the core body with appropriate gaps between them and attached thereto by brazing or the like (Japanese Patent No. 252057).
However, the core drill constructed such that the cutting blade chips including the diamond grains are attached on the periphery of the opening end of the core body is by far more expensive than other core drills, since expensive diamond grains are used in portions which are accommodated in the concave portions and do not perform cutting.
It is possible that the diamond grains are bound in a layered structure on an outer peripheral face, an inner peripheral face, and an end face of the opening end portion of the cylindrical core body. In that case, if such a core drill is used in drilling holes in concrete, stone, etc, the diamond grains in a region on which stress concentrate on tend to wear out and disengage from the core body in a relatively short time. As a result, the life of the core drill greatly decreases.
The present invention has been made under the circumstances, and an object of the present invention is to provide a cutting portion structure of a core drill which is capable of efficiently utilizing expensive diamond grains without wear or disengagement after a long-time use, and of thereby maintaining high cutting ability (drilling ability) for a long time period.